1. Field of the Invention
This invention relates to an automatic focus adjusting device for use in a camera or the like.
2. Related Background Art
A method of correcting an out-of-focus condition caused by movement of a moving object when the moving object is always pursued by an auto-focus (AF) system has already been proposed by the same assignee as the assignee of Japanese Patent Application No. 62-263728.
In the above-mentioned patent application, the movement of the image plane of the object is approximated to a quadratic function or a linear function while, on the other hand, the time required for distance measurement calculation, lens driving or release is foreseen under a certain assumption, and the position of the image plane of the object at a certain time in the future (for example, the time when the control of lens driving is completed or the time when the shutter curtains are moved after the release operation) is foreseen and in accordance with the result, lens driving is effected to thereby eliminate any pursuit delay relative to the moving object.
FIG. 2 of the accompanying drawings illustrates the lens driving correction method shown in the above-mentioned patent application. In the figure, the horizontal axis represents time t and the vertical axis represents the position d of the image plane of the object.
Curve f(t) indicated by a solid line represents the position of the image plane, at a time t, of the object which comes near the camera in the direction of the optic axis when the photo-taking lens is at infinity. Curve ((t) indicated by a broken line represents the position of the image plane of the object at the position of the photo-taking lens at the time t, and a section [t.sub.i, t.sub.i '] represents the focus detecting operation and a section [ti.sub.i ', t.sub.i+1 ] represents the lens driving operation. Accordingly, the difference in the direction of the vertical axis d between f(t) and l(t) at the same time t corresponds to the so-called defocus amount.
DFi represents the defocus amount detected at a time t.sub.i, DLi represents the amount of lens driving as converted into the amount of movement of the image plane executed from the result of the focus detection at a time t.sub.i-1, and TM.sub.i represents the time interval between the focus detecting operations.
In the example of the prior art shown in FIG. 2, the assumption that the position of the image plane of the object changes in accordance with a quadratic function is placed as a premise for correction. That is, it is assumed that if the current and past three positions of the image plane (t.sub.1, f.sub.1), (t.sub.2, f.sub.2) and (t.sub.3, f.sub.3) are known at a time t.sub.3, the position f.sub.4 of the image plane at a time t.sub.4 can be foreseen.
However, what the camera can actually detect are not the positions f.sub.1, f.sub.2 and f.sub.3 of the image plane, but the defocus amounts DF1, DF2, DF3 and the amount of lens driving DL1 and DL2 as converted into the amounts of movement of the image plane. The time t.sub.4 is a value in the future to the last, and is actually a value which varies as the accumulating time of a accumulation type sensor is varied by the brightness of the object, but here, for simplicity, it is assumed as known in the relation that t.sub.4 -t.sub.3 =t.sub.3 -t.sub.2.
Under the above-described assumption, the amount of lens driving DL3 when lens driving is effected toward t.sub.4 at a time t.sub.3 ' from the result of the focus detection at the time t.sub.3 is found from the following equations: ##EQU1##
If in FIG. 1, a point l.sub.1 is considered to be the origin, EQU f.sub.1 =DF1, f.sub.2 =DF2+DL1, f.sub.3 =DF3+DL2+DL1 (3) EQU t.sub.1 =0, t.sub.2 =TM1, t.sub.3 =TM1+TM2 (4)
If the equations (3) and (4) are substituted into the equations (2), (2)' and (2)' to find a, b and c, ##EQU2## Consequently, the amount of lens driving DL3 as converted into the amount of movement of the image plane at the time t.sub.4 is: ##EQU3##
Here, TM3 is assumed as known in the relation that TM3=TM2 as previously described, and DL3 is found from the equation (8).
The amount of lens driving at t.sub.n after the time t.sub.4 can likewise be found from the past three detected defocus amounts DF.sub.n-2, DF.sub.n-1 and DF.sub.n, the past two actual amounts of lens driving DL.sub.n-2 and DL.sub.n-1 and the past two time intervals TM.sub.n-2 and TM.sub.n-1. ##EQU4## If in accordance with the equations (8), (9) and (10), the defocus amount DL.sub.n for effecting lens driving is found from the detected defocus amount DF.sub.n and lens driving is effected, proper focusing even to a moving object will always become possible when lens driving is completed.
Now, the operation when the release operation has taken place during such automatic focus adjustment control will be described with reference to FIGS. 3 and 4 of the accompanying drawings.
FIG. 3 shows a case where the release operation has taken place at a time t.sub.x1 under the situation that focus detection is started at a time t.sub.n and lens driving of DL.sub.n is effected at t.sub.n ' and lens driving is completed at t.sub.n+1. Here, the time from after the release operation has taken place until film exposure is actually effected, i.e., the so-called release time lag, is TR. Thus, in the figure, film exposure is effected at a time t.sub.x1 +TR. In the case where lens driving is stopped simultaneously with the taking-place of the release operation, the position l.sub.x1 of the image plane of the lens at the time t.sub.x1 is the position l.sub.r1 of the image plane of the lens at the time t.sub.x1 +TR and at this time, the image plane of the object lies at f.sub.r1 and therefore, the object image exposed on the film suffers from defocus, i.e., out-of-focus, of f.sub.r1 -l.sub.r1 =d.sub.x1.
In the case where lens driving is continued even if the release operation takes place, l.sub.n+1, is reached at the time t.sub.n+1, and the position of the image plane of the lens at the time t.sub.x1 +TR is l.sub.r1 ', and although small in amount, out-of-focus of f.sub.r1 -l.sub.r1 '=d.sub.x ' still occurs.
FIG. 4 shows a case where the release operation has taken place during lens driving. As in the case of FIG. 3, when lens driving is stopped simultaneously with the release operation, out-of-focus of f.sub.r2 -l.sub.r2 =d.sub.x2 occurs, and when lens driving is terminated simultaneously with the release operation, out-of-focus of f.sub.r2 -l.sub.r2 '=d.sub.x2 ' occurs.
A description will now be given of a correction method which takes a uniform release time lag into account. In this case, the time t.sub.n+1 can be considered to extend by an amount corresponding to the release time lag TR and therefore, the equation (10) is modified as follows: ##EQU5##
FIG. 5 of the accompanying drawings shows the control method of the above equation (11). Curve f'(t) indicated by a dot-and-dash line represents the position of the image plane of the object which takes the uniform release time lag TR into account, and the lens can be controlled so as to be along this curve. Accordingly, the object in the viewfinder always becomes out of focus by an amount corresponding to the release time lag. Assuming that as in FIG. 3, the release operation has taken place at the time t.sub.x1, if lens driving is stopped, the position of the image plane of the lens is l.sub.r1 at the time t.sub.x1 +TR and the actual position of the image plane of the object is f.sub.r1 and therefore, out-of-focus of f.sub.r1 -l.sub.r1 =d.sub.x1 occurs. When lens driving is terminated, out-of-focus of f.sub.r1 -l.sub.r1 '=d.sub.x1 ' occurs. FIG. 6 of the accompanying drawings shows a case where the release operation has taken place during lens driving, and if lens driving is stopped simultaneously with the release operation, out-of-focus of f.sub.r2 -l.sub.r2 =d.sub.x2 occurs, and if lens driving is terminated simultaneously with the release operation, out-of-focus of f.sub.r2 -l.sub.r2 '=d.sub.x2 ' occurs.
As described above, even in the aforedescribed method which takes the release time lag into account, more or less out-of-focus remains depending on the timing of the release, but considerably good correction can be accomplished and this method sufficiently provides practical use. However, in the above-described correction method, it is assumed when DL3 is found in the equation (8) that the next focus detecting operation time interval TM3 is equal to the past focus detecting operation time interval TM2, but TM3 and TM2 consist of the defocus calculation time and the lens driving time, and although the defocus calculation time is substantially invariable, the lens driving time differs depending on the amount of lens driving and therefore, it cannot simply be assumed that TM3=TM2. Consequently, assuming that TM3=TM2, an error occurs to the foreseeing of the time when lens driving is completed and as a result, a correction error occurs. This phenomenon will now be described in detail with reference to the drawings.
FIG. 7 newly depicts the state of the first and subsequent focus detecting operations when the correction system shown in FIG. 5 or 6 is applied. In this figure, an ideal state in which the lens driving time does not depend on the amount of driving but is assumed to be always constant is assumed.
From the defocus amounts DF1, DF2 and DF3 obtained at times t.sub.1, t.sub.2 and t.sub.3 the amounts of lens driving DL1 and DL2 and TM1 and TM2, a.sub.3 and b.sub.3 are determined by the use of the equations (8) and (9) and DL3 is calculated by the use of the equation (11), and if lens driving is effected thereafter, the lens arrives at l.sub.4 at a time t.sub.4. When the release signal comes at this point of time, release takes place after TR, and since at this time, the image plane of the object is at f.sub.r4, it coincides with the lens position l.sub.4 and a photograph which is in focus can be taken. If the release signal does not come, the aforedescribed focus detecting operation cycle is repeated and the lens positions after the fourth and fifth focus detecting operations are l.sub.5 and l.sub.6, respectively.
However, the amount of lens driving DL3 is considerably great relative to DL2 and in the actual lens, the time required for driving the lens by DL3 unavoidably becomes longer than the time required for driving the lens by DL2 and accordingly, the true TM3 ought to be longer than what is estimated as TM3=TM2.
Likewise, comparing DL3 with DL4, DL4 is considerably smaller than DL3 and therefore, again, if it is assumed that TM4=TM3, an estimation error occurs to TM4. FIG. 8 shows that state.
From the defocus amounts DF1, DF2 and DF3 obtained at the times t.sub.1, t.sub.2 and t.sub.3 and the amounts of lens driving DL1 and DL2, a.sub.3 and b.sub.3 are determined by the use of the equations (8) and (9), and on the assumption that TM3=TM2, the amount of lens driving DL3 to the position f.sub.r4 of the image plane of the object which takes the release time lag TR into account is calculated by the use of the equation (11). Then, lens driving is started from t.sub.3 '. At this time, the lens is expected to arrive at l.sub.4, but since DL3 is great relative to DL2, more time than estimated at first is required and the lens arrives at l.sub.4 ' at the time t.sub.4. Consequently, a deviation of d.sub.x4 occurs relative to the target position f'(t) at the time t.sub.4.
When the next focus detection is effected at the time t.sub.4 and the defocus amount DF4 is obtained, the focus detection time interval is now assumed to be TM4=TM3'. That is, the real value TM3' of the last interval is utilized. Then, the position f.sub.r5 of the image plane of the object which takes TR into account is foreseen and driving by DL4 is effected. Now that DL3&lt;DL4, driving is completed in a time shorter than estimated at first and that time is t.sub.5. At this time, a deviation of d.sub.x5 occurs.
As described above, the amount of lens driving differs each time, and particularly before and after corrected driving is entered, a great variation occurs to the amount of driving. Consequently, from the difference in the lens driving time, an estimation error occurs to the focus detection time error and as a result, correction accuracy is somewhat reduced.